دانلود رایگان مقاله مکانیزم ایزوتوپ پایدار کلر در حوزه غیر دریایی خشک

abstract

Stable chlorine isotopes are useful geochemical tracers in processes involving the formation and evolution of evaporitic halite. Halite and dissolved chloride in groundwater that has interacted with halite in arid non-marine basins has a d37Cl range of 0 ± 3‰, far greater than the range for marine evaporites. Basins characterized by high positive (þ1 to þ3‰), near-0‰, and negative (0.3 to 2.6‰) are documented. Halite in weathered crusts of sedimentary rocks has d37Cl values as high as þ5.6‰. Saltexcluding halophyte plants excrete salt with a d37Cl range of 2.1 to 0.8‰. Differentiated rock chloride sources exist, e.g. in granitoid micas, but cannot provide sufficient chloride to account for the observed data. Single-pass application of known fractionating mechanisms, equilibrium salt-crystal interaction and disequilibrium diffusive transport, cannot account for the large ranges of d37Cl. Cumulative fractionation as a result of multiple wetting-drying cycles in vadose playas that produce halite crusts can produce observed positive d37Cl values in hundreds to thousands of cycles. Diffusive isotope fractionation as a result of multiple wetting-drying cycles operating at a spatial scale of 1e10 cm can produce high d37Cl values in residual halite. Chloride in rainwater is subject to complex fractionation, but develops negative d37Cl values in certain situations; such may explain halite deposits with bulk negative d37Cl values. Future field studies will benefit from a better understanding of hydrology and rainwater chemistry, and systematic collection of data for both Cl and Br. ©

6. Conclusions

1. The range of d37Cl in lacustrine halite (including salty mudstone deposits) is broad, 3 to þ3‰. Some halite deposits (e.g. western and central China) have only d37Cl values near 0‰, while others (western China, western USA) are characterized by more extreme positive d37Cl values (>þ1‰), and, in two cases (western China and Chile), negative values. 2. The range of d37Cl in dissolved chloride in groundwater that has interacted with halite deposited in arid environments is also broad, 1 to þ2.4‰. In two cases, the Deep River basin (North Carolina) and China Lake (California), values of d37Cl >þ1‰ are interpreted to represent halite-bearing mudstone and fossil evaporite brine, respectively. 3. Halite with d37Cl values ranging from þ0.7 to þ5.5‰ is present in weathered crusts of salty Neogene sedimentary rock in Safford Basin, Arizona. In chloride samples representing unweathered rock of Safford Basin, the d37Cl range is 0 ± 0.7‰. 4. Strongly fractionated rock sources of chloride are unlikely to explain extreme d37Cl values in continental lacustrine deposits. 5. Single-pass applications of known fractionating mechanisms (halite-solution equilibrium, diffusion) operating on ambient chloride with d37Cl near 0‰ are incapable of generating the more extreme (>1‰) observed d37Cl values reviewed here. 6. In vadose playas, small incremental changes in d37Cl, repeated over hundreds to thousands of annual cycles of wetting and evaporation to form halite crusts, and resulting from downward discharge of evaporite brine, can generate lacustrine salt with d37Cl values of þ2‰ or greater. Phreatic playas with no subsurface discharge will undergo no such isotope fractionation, whether or not halite is formed, 7. In the weathered crust of salty sedimentary rock, small incremental changes in d37Cl can result from outward diffusion of chloride depleted in 37Cl, leaving an enriched salt reservoir at depth within the crust. Diffusion at a spatial scale of 1e10 cm, and a time scale of days, is sufficient if repeated over many cycles of wetting and drying. 8. Cl isotope fractionation within halophyte plants that exclude salt during root uptake of water generates d37Cl values as low as 2‰ in salt excreted by the plants, but cannot lead to a lasting separation of distinct fractions of chloride. 9. Halite with negative d37Cl values (1‰) can at present be explained only by input of low-d37Cl chloride in precipitation. Th